iPhone 4 (5.1.1) Voice call problem after data session

Whenever I use data (EDGE / Wi-Fi), the handset becomes heated. If I initiate a voice call at that moment, the dialer starts with the 'speakerphone' icon being pressed. At that moment, no party can hear no party, even though in the set it shows the call is active. In this state sometimes the handset also hangs and requires a hard reset

However, after a minute or two (when not using data) the handset becomes cold again (quite fast) and then when I initiate a call, the 'speakerphone' icon is not being pressed anymore and everything is normal!

Really donno, what is actually creating this strange problem!! Overheating battery? Overheating wi-fi chip? Overheating any other chip? Firmware? or just my luck!!

P.S: To keep the phone operation smooth, I've disabled wifi & cellular data at the moment! And all is running well. So, it must be something with data usage!

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